Flexible trailing elements in a paper-making machine headbox having projections thereon extending into the slurry flow

ABSTRACT

1. In a headbox construction for delivering stock to a forming surface, the combination comprising: a slice chamber for receiving stock from a headbox chamber and being tapered in a downstream direction; a slice opening leading from the chamber for discharging stock flowing through the chamber onto a traveling forming surface; a plurality of flexible trailing sheet form elements each extending in a cross-machine direction and being separated to form flow passages therebetween; a plate member having stock flow openings therethrough located at the upstream end of the slice chamber and supporting the upstream ends of the trailing elements, the upstream ends of the trailing elements being anchored to said plate member and the downstream ends being unattached and self-positionable so as to be solely responsive to forces exerted thereon by the stock flowing toward the slice; and a plurality of projections on the surface of said trailing elements substantially uniformly space thereon projecting int the flow steam which flows fast the trailing elements increasing the surface area of the trailing elements exposed to the stock flow stream.

Oct. 22,1974 R. A. BETLEY ETAL 3,843,470 I FLEXIBLE TRAILING ELEMENTS IN A PAiERMAKING MACHINE HEADBOX HAVING PROJECTIONS THEREON EXTENDING INTO THE SLURRY FLOW Filed Nov. 21. 1972 4 Sheets-Sheet 1 R. A. BETLEY ETA!- Oct. 22, 1974 FLEXlBLE TRAILINGELEMENTS IN A PAPERMAKING MACHINE HEADBOX HAVING PROJECTIONS THEREON EXTENDING INTO THE SLURRY FLOW 4 Sheets-Sheet 2 Filed Nov. 21, 1972 lA/VENTORS 22 459271022 EQ fl/Z fl. 56

ATTORNEYS Oct. 22, 1974 BETl-EY 5"- 3,843,470

FLEXIBLE TRAILING EIJEMENTS 1N A PAPEHMAKING MACHINE HEADBOX HAVING PROJECTIONS THEREON EXTENDING INTO THE SLURRY FLOW Filed Nov. 21, 1972 Y 4 Sheets-Sheet 4.

United States Patent Oflice 3,843,470 Patented Oct. 22, 1974 3,843,470 FLEXIBLE TRAILING ELEMENTS IN A PAPER- MAKING MACHINE HEADBOX HAVING PRO- JECTIONS THEREON EXTENDING INTO THE SLURRY FLOW Raymond A. Betley and Ralph A. Beck, Beloit, Wis., assignors to Beloit Corporation, Beloit, Wis. Continuation-impart of abandoned application Ser. No. 68,350, Aug. 31, 1970. This application Nov. 21, 1972, Ser. No. 308,555

Int. Cl. D21f 1/02, N06

US. Cl. 162-343 12 Claims ABSTRACT OF THE DISCLOSURE A papermaking machine headbox which has a tapered slice chamber having multiple, flexible trailing elements therein extending in the cross-machine direction. The surfaces of the trailing elements are provided with projections which project into the slurry flow stream to increase the surface area of the trailing elements exposed to the stock stream. These projections may be in the form of ridges, or may be conical or chisel shaped. On the other hand the trailing elements may be in the form of corrugated plates. The downstream ends of the trailing elements are unattached leaving them self-positionable and solely responsive to forces exerted thereon by the stock flowing toward the slice opening.

BACKGROUND OF THE INVENTION The present application is a continuation-in-part of our copending application, Ser. 68,350, filed Aug, 31, 1970 entitled Headbox Slice Chamber," now abandoned.

The present invention relates to improvements in paper making machines and more particularly to an improved slice chamber construction with a self-positionable trailing element therein that has projections on the surface extending into the fiow stream so that the entire trailing element is free at its downstream end and positions itself in accordance with the forces exerted thereon by the stock.

A significant difficulty in achieving uniform formation of a paper web on a traveling forming surface is the natural tendency of the fibers to flocculate. An objective in fourdrinier machine designs, and particularly the headbox, has been to disperse the fiber networks during the period of flow through the headbox in such a manner that flocculation has the least tendency to occur on the forming wire surface. Within the headbox this is accomplished by dispersion of the fiber networks by generating turbulence. This has been done by constructions which generate and maintain the turbulence in the flow to the headbox and within the headbox such as by rectifier rolls and like structures, and in fiow through the slice chamber to the slice opening. Maintaining the turbulence generated within the headbox uniform across the headbox width is significant, particularly in machines wherein table rolls have been replaced by suction boxes, and the fiber suspensions drain more rapidly with considerably less activity generated in the undrained suspension on the forming wire. With the provision of large scale turbulence in the headbox, this must be maintained uniformly dispersed across the headbox in a direction parallel to the slice and transversely to the slice, and the turbulent energy is reduced to smaller scale turbulence in the flow through the slice to the slice opening.

It is accordingly an object of the present invention to provide an improved headbox construction and particularly an improvement in the slice chamber wherein high speed stock flow can be delivered to a forming surface and wherein uniform turbulence will be maintained across the headbox and large scale turbulence will be uniformly converted to small scale turbulence in flow from the primary chamber of the headbox to the slice opening.

A further object of the invention is to provide an improved headbox wherein the stock performance is controlled throughout the fiow through the headbox and the slice chamber in such a manner that it is delivered onto the forming surface with a minimum tendency of the fibers to flocculate and obtaining uniform formation of the paper web.

A still further object of the invention is to provide a slice chamber with self-positionable freely trailing elements therein constructed to maintain fine scale turbulence for improved fiber dispersion and distribution onto the forming surface.

A feature of the invention is the provision of a selfpositionable trailing element in a slice chamber having a nonsmooth surface wherein the element is self-positionable and the nonsmooth surface contributes to the maintenance of fine scale turbulence in the stock and wherein the resistance of the projections of the nonsmooth surface create lateral forces in the trailing elements to help control their position in the slice chamber.

Other objects, advantages and features, as well as equivalent structures which are intended to be covered hereby, will become more apparent with the teaching of the principles of the invention in connection with the disclosure of the preferred embodiments in the specification, claims and drawings, in which:

DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic longitudinal sectional view taken through a slice chamber constructed in accordance with the principles of the present invention;

FIG. 2 is a view looking substantially along line II--II of FIG. 1, illustrating a part of a form of trailing element;

FIG. 3 is a side view of the trailing element substantially along line III-III of FIG. 2 and generally illustrating the warp and weft shoot strands of the element which is diagrammatically illustrated in FIG. 2;

FIG. 4 is a view somewhat similar to FIG. 3 but illustrating a modified form of element in which the weft shoot strands decrease in diameter as they approach the slice lip;

FIG. 5 is a longitudinal sectional view taken through a slice chamber constructed in accordance with the principles of the present invention and showing a still further modified form;

FIG. 6 is a generally perspective view showing a pair of corrugated elements, one spaced vertically from the other;

FIG. 7 is a generally diagrammatic perspective view showing a pair of elements in which the elements are corrugated and the corrugations of one element extend laterally of the element and the corrugations of a next adjacent element extend diagonally across the element;

FIG. 8 is a view somewhat similar to FIG. 7, but illustrating a modified form and arrangement of elements, from those shown in FIGS. 6 and 7;

FIG. 9 is an enlarged view of an alternative method for mounting the elements in the plates;

FIG. 10 is a fragmentary vertical sectional view taken substantially along line XX of FIG. 9;

FIG. 11 is a fragmentary sectional view, viewed looking in an upstream direction, showing another form of fiexi ble trailing element;

FIG. 12 is a fragmentary perspective view showing another form of trailing element; and

FIG. 13 is a fragmentary perspective view showing a further form of trailing element.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 generally shows in schematic form a forming wire F traveling around a breast roll 11, to define a con 3 ventional forming surface onto which paper stock is fed through a slice Opening generally at S. The slice opening S is at the discharge end of a tapered slice chamber 12 which is a prcslice chamber 13 of a headbox (not shown),

and aligning the stock for flowing through the slice chamber 12. The stock is generally fed to the headbox through a slice inlet from a pump or other suitable source of stock v(not shown) in a conventional manner, which is no part of the present invention, so need be shown or described herein.

The slice outlet S is formed by two slice plates 15 and 16, pivoted to the forward ends of converging top and bottom plates 17 and 18 respectively, converging from a perforated plate 19 toward the slice opening S. The plates 15 and 16 may be respectively pivotally, moved about their axes of connection to the top and bottom plates 17 and 18, to accommodate adjustment of the slice opening, and may be adjustably moved relative to each other and held in position by suitable adjustment mechanisms, which may be worm and worm gear adjustment devices 20 and 21, or may be fluid pressure operated devices of a conventional form (not shown).

The perforated plate 19 extends transversely of the preslice chamber 13 of the headbox and contains a plurality of generally horizontal rows of perforations 23 with land areas 24 therebetween. The width of the land areas depends upon the open area of the plate required for a particular stock flow. In order to prevent plugging of the plate 19, it is desirable to maintain the overall open area of the plate 19 greater than 30%. The openings in the perforated plate, however, should be as small as possible to maintain the required flow pattern, but should be large enough to avoid plugging and may vary with a particular application and stock characteristics.

Extending through the land areas 24 are a plurality of flexible sheet form nonsmooth trailing elements 25, extending for the width of the slice chamber 12 to the sides thereof and preferably free from the sides of the slice chamber. The nonsmooth trailing elements 25 may be aflixcd to the perforated plate 19 in a suitable manner and are freely floating on the downstrtam side of said plate and converge toward each other as they approach the slice opening. The trailing elements thus form a plurality of channels 22, creating the turbulence for fiber dispersion and positioned in spaced relation with respect to each other by the hydrodynamic effects of stock flow.

The sheet form trailing elements 25 are shown in FIGS. 2, 3 and 4 as being in the form of fourdrinier wires or other suitable woven material having weft shoot strands 26 which may be in the general form of rods and wire warp strands 27. In FIGS. 2 and 3 the weft shoot strands 26 of the wire are shown as being of a uniform diameter for the length of the wire and preferably extend transversely of the slice chamber to provide uniform flow suppressing elements extending for the length of the slice chamber. The warp strands may be of the same diameter throughout the length of the wire and extend longitudinally of the wire.

The trailing elements 25 also project upstream of the perforated plate 19 and have rods 28 extending across the rear end portions thereof, serving to prevent the stapling of fibers to the upstream ends of said elements and to the upstream sides of the perforated plate 19. The rods themselves should be large enough in diameter to pre vent the stapling of fiber thereto and may be of various shapes, such as cylindrical, flat, blunt or tear-drop.

By the use of a woven material as a trailing element, materials can be selected to suppress the flow and create the turbulence for a particular stock required to make the form of paper desired. The woven design has the abiilty to expand without buckling and may be thermal set, although it need not necessarily be so set. The material can also be coated.

In FIG. 4 we have shown a modification of the invention in which the size of the weft shoot strands 26 is varied to get the thicker section of the trailing element at the upstream end thereof and a thin section of the element at the discharge tip of the element. The weft shoot strands 26 may vary in size in sections A, B, C, D, E and F and so forth as needed, to obtain the thickness required for the length of the element. The elements can thus be arranged to obtain a smooth transition from the upstream to the downstream end thereof. The warp strands 27 of a given element may be of the same diameter throughout the length of the woven element but may vary in diameter from one element to the other. Moreover, one element may be tapered from its upstream to its downstream end while a next adjacent element may have a different taper, or may be of a uniform thickness from its upstream to its downstream end.

The structure just described thus provides a plurality of individual channels 22 between the trailing elements 25, floating in the slice chamber and converging from the upstream to the downstream end thereof, which may be of uniform thickness throughout the length of the chamber or may converge from the upstream to the downstream ends thereof. The flexibility of the wire achieves a uniform spacing between the flow suppressing elements across the width of the slice chamber, positioned and maintained by the hydrodynamic effects of stock flow, maintaining a stable condition of the particular trailing elements in which the elements may flex to allow for the passage of large particles of stock in the stock flow system.

In FIGS. 5, 6, 7 and 8 we have shown a modified form in which our invention may be carried out. In these Figures the textured trailing elements are in the forms of corrugated plates. The slice chamber is the same as that illustrated in FIG. 1. The variable slice lip is also the same and the slace opening S cooperates with the breast roll for laying stock on the wire in the same manner as the slice opening in FIG. 1. Like part numbers, therefore, will be applied to the same parts as were applied in the form of the invention illustrated in FIG. 1.

In FIG. 5 a perforated plate 29 is shown as extending across the slice chamber 12 and as defining the upstream end of said slice chamber. The plate 29 has rows of perforations 30 with lands 31 therebetween and forming mountings for a plurality of textured elements in the form of corrugated elements 32, which may be corrugated plates. As shown in FIG. 5, each trailing element is clamped at its upstream end to a rod 33 carried in a socket in the land 31, to accommodate free floating movement of the element, and to enable the flow of stock to hydrodynamically position the elements in the required spaced relation relative to each other.

The corrugated elements 32 may be made from thin stainless steel plates, molded plates or synthetic plates made from any suitable material, thin enough to provide the flexibility required to enable the elements to be floatingly mounted in the slice chamber and positioned by the hydrodynamic effects of stock flow, and to make it unnecessary to secure the elements to the sides of the slice chamber to maintain the proper spacing of the elements.

As shown in FIG. 6, the corrugations of one trailing element extend transversely of the element and slice chamber while the corrugations of the next adjacent trailing element extend longitudinally of the element. The corrugations of each element, however, may extend transversely or longitudinally of the element or diagonally across the element.

In FIG. 6, the corrugations extending transversely of the element 32 are shown as extending in a machine cross direction and are rather large at the upstream ends of the elements to give a gross turbulent effect as stock enters the slice chamber and diminish to reduce the turbulence to the micro-turbulence desired at the slice opening.

In FIG. 7 we have shown an element 32 with the corrugations extending in a cross machine direction and a next downwardly spaced flow suppressing element 32 with the corrugations extending diagonally across the slice chamber from one side thereof to the other.

In FIG. 8, the corrugations of the How suppressing elements 32 extend diagonally across the slice chamber in one direction while the corrugations of a next downwardly spaced trailing element 32 extend diagonally across the slice chamber in an opposite direction. In these arrangements of trailing elements, the corrugations are relatively high at the upstream end of the trailing element to give a gross turbulent effect and diminish to relatively fine corrugations at the downstream end of the flow suppressing elements to give the required low turbulence to the stock as it passes through the stock opening onto the fourdrinier wire F.

The trailing elements 32 similar elements 25 are resilient enough to float freely within the chamber, and form a plurality of longitudinally extending channels through which the paper making stock will flow, maintained in spaced relation by the hydrodynamic effects of stock flow, and gradually reduce large scale turbulence in the paper making stock, but maintain fine scale turbulence for fiber dispersion.

In FIG. 9, the elements are mounted in an alternative manner which provides certain advantages during operation of the present invention. Oftentimes, when the elements are subjected to an elevated temperature during the use thereof, the material from which they are made expands at a different rate than the expansion of the plate in which they are mounted. If no provision is made to release the tension between the two materials, stresses will be transmitted to the elements which will deform in a manner not desired. Accordingly, the means described in FIG. 9 may be employed to mount tthe elements while still providing for free thermal expansion.

A perforated plate 35 extends across the slice chamber as in the manner plate 29 of FIG. 5 does, having perforations 40 and lands 41 therebetween. Lands 41 form a plurality of mountings for the plurality of textured elements 36. Each element 36 is mounted in land area 41 as shown in FIG. 9. A dove tail groove 42 is machined into the land area 41 of plate 35 such that the wide end of the dove tail groove 42 is upstream from the element 36. Mounting the element 36 in the dove tail groove 42 is a compressible clamp 37 which normally will be made from the same material as the elements 36. This is not essential, however, since the element 36, the clamp 37 and the land area 41 are free to move independent of each other when subjected to thermal expansion. Raised areas of the clamp 37 contact the element 36 to grip the element 36 when forces tend to push the element 36 in a downstream direction. If the coefliciencies of friction between the raised area 38 of the clamp 37 and the element 36 is not sufficient, such as when the element 36 may be coated with Teflon or other low friction materials, staples 39 may be inserted in element 36 just slightly upstream of the portion 38 of the clamp 37. Thus when thermal expansion acts on the various parts of the device, they are free to expand independently and no stress is transmitted to the elements 36.

FIG. illustrates an arrangement wherein the trailing elements 36 are in the form of individual narrow flat strips or ribbons 50. The ribbons are separated from each other and each have a plurality of projections such as 51 and 52 on their surfaces.

FIG. 11 illustrates another form of trailing elements wherein they are in the form of individual filaments 53. The filaments have projections 54 on their surface. The projections will be spaced from each other in a circumferential direction and also spaced in an axial direction. In the arrangement of FIG. 10, the spacing of the projections 51 and 52 will cause them to be dispersed in a crossmachine direction as Well as in a machine direction, and they may be arranged in rows or may be randomly spaced.

In the arrangement of FIG. 12, the trailing element 55 has projections 56 which are pointed. The pointed projections have a sloped upstream portion 56a and a sloped downstream portion 56b, and the projections may be chisel shaped or may be conically shaped.

FIG. 13 illustrates another form of trailing member 57. The projections are in the form of ridges 58 and 59 which extend in a cross-machine direction.

The preferred form of arrangements are shown in. FIGS. 10 through 13 wherein the projections are on the surface of a flexible trailing element. It will be understood that while flexible elements are preferred, a rigid trailing element may be in some instances be employed with the rigid element pivotally anchored at its upstream end and unattached at its downstream end so as to be pivotally free floating.

In the arrangement where the nonsmoothness of the surface is obtained by projections such as shown in FIGS. 10 through 13, the projections may be separately attached to a smooth surface. For example, in the arrangements of FIGS. 10 or 12, the trailing element may be formed of plastic, and the projections cemented to the surface. A material such :as Lexan can be formed in the shape of the projections required and cemented to the surface of the plastic flexible sheet.

In the arrangement of FIG. 13, the projections 58 and 59 may be cylindrical rods which are slit in half, with each half being cemented to the surface of the sheet to form the cross-machine extending ridge. The structure of FIG. 13 may also be formed by an extrusion arrangement wherein the ridges are integral with the sheet at the time the sheet is extruded from a plastic extruder.

Thus, it will be observed from the foregoing description that the nonsmooth surface may be obtained by projections of various forms on the sheets or filaments. The projections should have a depth, i.e., a height in the range of from .02" to .5". They may be regularly spaced or random spaced with the spacing ranging from .1" to 10" between adjacent projections.

A suitable arrangement for the construction of FIG. 13 will provide projections A" in height spaced apart from each other 2" to 5".

Thus, the nonsmooth surface shall mean 'a surface of a sheet or filament having projections in the range above expressed, and the projections are sutficient to have an effect on the flowing stock stream. While the projections may be randomly positioned, they should be uniform on both sides of the sheet or filament so as to eliminate the possibility of flutter. The size of the projections, or in other words, the size of the surface roughness can be changed to create a lesser or greater activity for fiber dispersion. The amount of turbulence will depend upon the velocity of flow which will be dictated by the requirements of the paper machine.

We claim as our invention:

1. In a headbox construction for delivering stock to a forming surface, the combination comprising:

a slice chamber for receiving stock from a headbox chamber and being tapered in a downstream direction;

a slice opening leading from the chamber for discharging stock flowing through the chamber onto a traveling forming surface;

a plurality of flexible trailing sheet form elements each extending in a cross-machine direction and being separated to form flow passages therebetween;

a plate member having stock flow openings therethrough located at the upstream end of the slice chamber and supporting the upstream ends of the trailing elements,

the upstream ends of the trailing elements being anchored to said plate member and the downstream ends being unattached and self-positionable so as to be solely responsive to forces exerted thereon by the stock flowing toward the slice;

and a pluarilty of projections on the surface of said trailing elements substantially uniformly spaced 7 thereon projecting into the flow stream which flows past the trailing elements increasing the surface area of the trailing elements exposed to the stock flow stream.

2. In a headbox construction for delivering stock to a forming surface constructed in accordance with claim 1:

wherein the projections are in the form of laterally extending continuous ridges on the surface of the trailing elements with the ridges spaced from each other in a downstream direction and projecting from both surfaces of the trailing elements.

3. In a headbox construction for delivering stock to a forming surface constructed in accordance with claim 1:

wherein the projections have a height in the range of 4. In a headbox construction for delivering stock to a forming surface constructed in accordance with claim 1:

wherein the projections are spaced apart in a downstream direction in the range of from .1" to 10.0".

5. In a headbox construction for delivering stock to a forming surface, the combination comprising:

a slice chamber for receiving flowing fibrous stock from the upstream end and having an opening on the downstream end for discharging stock onto a traveling forming surface;

a flexible trailing element positioned in the slice chamher;

and means anchoring the upstream end of said element upstream in said slice chamber,

the downstream end being unattached and constructed to be self-positionable in the slice chamber being solely responsive :to forces exerted thereon by the stock flowing toward the slice opening;

and a plurality of projections on the surface of the trailing element projecting laterally into the stock flow stream in the slice chamber increasing the surface area of the trailing elements exposed to the stock flow stream.

6. In a headbox construction for delivering stock to a forming surface constructed in accordance with claim 5:

wherein a plurality of flexible trailing elements are positioned in the slice chamber each being in sheet form and extending laterally in a direction across the slice chamber and the projections are in the form of ridges extending laterally in a direction across the slice chamber with the ridges positioned on the surfaces of the trailing elements.

7. In a headbox construction for delivering stock to a forming surface constructed in accordance with claim 5: wherein the projections diminish in size in a downstream direction.

8. In a headbox construction for delivering stock to a forming surface constructed in accordance with claim 5: wherein the flexible trailing element is in a substantially sheet form extending laterally in a direction across the slice chamber.

9. In a headbox construction for delivering stock to a forming surface constructed in accordance with claim 8:

wherein the projections are pointed and have an inclined surface on the upstream end and an inclined surface on the downstream end with the surfaces leading toward the point of the projection.

10. In a headbox construction for delivering stock to a forming surface constructed in accordance with claim 8: wherein the projections have a rounded outer surface extending into the flow stream.

11. In a headbox construction for delivering stock to a forming surface, the combination comprising:

a slice chamber for receiving stock from a headbox chamber and being tapered in a downstream direction;

a slice opening leading from the chamber for discharging stock flowing through the chamber onto a traveling forming surface;

a plurality of flexible trailing sheet form elements each extending in a cross-machine direction and being separated to for-m flow passages therebetween;

a plate member having stock flow openings therethrough located at the upstream end of the slice chamber and supporting the upstream ends of the trailing elements,

the upstream ends of the trailing elements being anchored to said plate member and the downstream ends being unattached and self-positionable so as \to be solely responsive to forces exerted thereon by the stock flowing toward the slice;

and a plurality of projections on the surface of said trailing elements substantially uniformly spaced thereon into the flow stream which flows past the trailing elements, said trailing elements being in the form of open woven screens with the projection formed by the material of the screens and with the trailing elements having openings theret-hrough.

12. In a headbox construction for delivering stock to a forming surf-ace, the combination comprising:

a slice chamber for receiving stock from a headbox chamber and being tapered in a downstream direction;

a slice opening leading from the chamber for discharging stock flowing through the chamber onto a traveling forming surface;

a plurality of flexible trailing sheet form elements each extending in a cross-machine direction and being separated to form flow passages therebetween;

a plate member having stock flow openings therethrough located at the upstream end of the slice chamber and suporting the upstream ends of the trailing elements,

the upstream ends of the trailing elements being anchored to said plate member and the downstream ends being unattached and self-positionable so as to be solely responsive to forces ex erted thereon by the stock flowing toward the slice;

and a plurality of projections on the surface of each of said trailing elements substantially uniformly spaced thereon projecting into the flow stream which flows past the :trailing elements, said flexible trailing elements each being in a sheet form extending laterally in a direction across the slice chamber, and the trailing elements each being formed of a woven screen.

References Cited UNITED STATES PATENTS 3,607,625 9/1971 Hill et al. 162343 FOREIGN PATENTS 997,539 7/1965 Great Britain l622l6 S. LEON BASHORE, Primary Examiner R. H. TUSHIN, Assistant Examiner US. Cl. X.R. 162-336. 344 

1. In a headbox construction for delivering stock to a forming surface, the combination comprising: a slice chamber for receiving stock from a headbox chamber and being tapered in a downstream direction; a slice opening leading from the chamber for discharging stock flowing through the chamber onto a traveling forming surface; a plurality of flexible trailing sheet form elements each extending in a cross-machine direction and being separated to form flow passages therebetween; a plate member having stock flow openings therethrough located at the upstream end of the slice chamber and supporting the upstream ends of the trailing elements, the upstream ends of the trailing elements being anchored to said plate member and the downstream ends being unattached and self-positionable so as to be solely responsive to forces exerted thereon by the stock flowing toward the slice; and a plurality of projections on the surface of said trailing elements substantially uniformly space thereon projecting int the flow steam which flows fast the trailing elements increasing the surface area of the trailing elements exposed to the stock flow stream. 